The capture and sequestration of carbon dioxide (CO2) emissions needs to be significantly improved if the climate change consequences of such emissions are to be controlled or curtailed. The CO2 produced from combustion and industrial processes, specifically power plant flue gas, is perhaps the largest single greenhouse gas emission. Most existing carbon capture and sequestration methods take a two-step approach. First, a method is sought for separating CO2 from the flue gas or other gaseous emission source. These may include capture of the CO2 in liquid solvents, solid zeolyte or various membranes. However, the capture media need to be regenerated without releasing the CO2 into the atmosphere, and this is difficult to achieve in standard physical separation processes.
The second step is sequestering the CO2 gas or liquid by inserting it into underground geological formations or in deep ocean layers. However, very specific geological configurations are required for disposal of the CO2, and these are not commonly available at CO2 emission sites. Thus, transportation adds substantial cost and difficulty. In addition, it is not known whether CO2 can be permanently sequestered underground. The two-step approach also is not economical because often CO2 represents only a small percentage of a large volume of flue gas, and treating a large flow stream to recover a small portion of it as CO2 is wasteful and expensive.
Another approach to CO2 capture and sequestration involves mining, crushing and transporting rocks to the emission site, where the crushed rock is used to absorb CO2. But this requires a good deal of heat and pressure. The energy input and environmental costs of mining the rock and transporting it to and from the CO2 source, as well as the energy costs of having the crushed rock accept and absorb the CO2, are very high.
Other ways to capture CO2 include chemical absorption using liquids such as amines or aqueous solutions of bases, physical absorption in an appropriate solution and membrane separation. All of these methods have the problem that the absorption media need to be regenerated without losing CO2. Other capture methods such as physical adsorption and cryogenic separation require significant amounts of energy in the form of heat or pressure.
Some CO2 capture methods react CO2 (or carbonic acid formed from water and CO2) with an aqueous solution of an alkali to form a carbonate. However, a significant drawback of that approach is that the carbonate exits the process in solution with water, requiring further, energy intensive treatment to separate the solids and the water, or it results in a large volume, heavy, wet, cement-like paste that requires energy intensive drying and mechanical systems to control the size, configuration and weight of the resulting dried product.
Although some are examining techniques for capturing and sequestering CO2 from ambient air, they are not suitable for CO2 emissions from power plants because of the substantial difference in CO2 concentration between ambient air and flue gas. Ambient air generally contains between about 0.03% and 0.04% CO2, whereas flue gas contains 3.0% or higher concentrations of CO2. Removing very small quantities of CO2 from the very large quantities of ambient air is not as viable and as productive as the capture and sequestration of large amounts of CO2 from streams, such as flue gas, where the CO2 is more concentrated.
Therefore, there exists a need for a commercially viable carbon capture and sequestration process that works at industrial scales and is complete and permanent. Specifically, there is a need for a carbon capture system that does not use capture media that require complex and energy-intensive regeneration, and does not yield a heavy, wet end product that requires energy intensive drying and other post-capture processing. There is a further need for a carbon capture and sequestration process that permanently sequesters CO2 at the site of CO2 emission. In summary, a need exists for a carbon capture and sequestration system that is cost effective and not energy intensive and results in permanent sequestration of CO2.